This invention relates to semiconductive ceramics for use in ceramic heaters, ceramic sensors, resistor substrates and the like, and a supporting member for supporting a magnetic disk substrate.
In general, ceramics is an insulating material. Recently, however, there have been marketed semiconductive ceramics having the property of semiconductivity. Silicon carbide ceramics or Perovskite ceramics, such as lanthanum chromite, have been conventionally used as semiconductive ceramics for use in ceramic heaters, ceramic sensors or the like.
Also, there have been proposed semiconductive structural ceramics in which conductive material, such as metal oxides, metal nitrides, or metal carbides, is added in a conventional structural ceramics. For example, Japanese Unexamined Patent Publications Nos. (HEI) 2-295009 and 1-243388 respectively disclose alumina semiconductive ceramics and zirconia semiconductive seramics which are obtained by mixing TiO.sub.2, TiC, NiO, or CoO as conductive material with alumina material or zircona material, and sintering the mixture in a reducing condition.
Further, there have been recently proposed use of a semiconductive ceramics as a supporting member for supporting a magnetic disk substrate in a magnetic disk unit.
As shown in FIG. 1, specifically, a magnetic disk unit 50, which is used as an external storage means of a computer, comprises a hub 14 fixedly attached to a rotary shaft 13, and magnetic disk substrates 15 placed on the periphery of the hub 14, and ring-shaped spacers 11 placed on the periphery of the hub 14. The spacers 11 are placed between the magnetic disk substrates 15. Further, the magnetic disk substrates 15 and the spacers 11 are pressed with a ring-shaped shim 10 and a cramp 12, and tightly fastened on the hub 14 by screws 16. Indicated at 17 are magnetic heads.
The rotary shaft 13 is rotated by a driver. The magnetic disk substrates 15 rotates together with the rotation of the rotary shaft 13 without coming into contact with the magnetic heads 17 so that information is written and read at a specified position of a magnetic disk substrate 15.
The magnetic disk unit 50 has been demanded to store a large amount of information at a higher density. Accordingly, it is required to make the distance between the magnetic head 17 and the magnetic disk substrate 15 shorter, and increase the flatness and the smoothness of the magnetic disk substrate 15. For this reason, the magnetic disk substrate 15 has been proposed to be made of glass material because the glass material makes it possible to ensure a higher surface flatness and smoothness. In this case, the supporting member for supporting the magnetic disk substrate 15, that is, the spacer 11, the shim 10, and the clamp 12, are made of ceramics or glass in order to prevent the magnetic disk substrate 15 from deforming due to a difference in thermal expansion between the magnetic disk substrate 15 and the supporting member. Japanese Examined Patent Publication No. (HEI) 5-80745 and Japanese Unexamined Patent Publication No. (SHO) 61-148667 disclose such a technique.
However, the conventional supporting member is made of insulating glass or ceramics. Recently, it has been revealed that the magnetic disk substrate 15 held by the supporting member made of insulating ceramics or glass has an electric charge, consequently causing damages in the writing and reading of information on the magnetic disk substrate 15. To solve this problem, a film of metal such as aluminum or zinc has been proposed to be coated in the surface area of the supporting member that comes into contact with the magnetic disk substrate 15 to ground the electric charge.
However, the provision of the metal film decrease the flatness of the contact surface of the supporting member, consequently causing a strain in the magnetic disk substrate 15, and further a likelihood that the magnetic head 17 comes into contact with the magnetic disk substrate 15 to damage it. In addition, there is a likelihood that the metallic film peels off from the supporting member due to a thermal expansion difference relative to the glass or ceramics supporting member, then cutting off the grounding of electric charge in the magnetic disk substrate 15.
In view of these problems, Japanese Unexamined Patent Publication No. (HEI) 2-226566 proposes a technique of making a supporting member of semiconductive ceramics to prevent electric charge from accumulating in the magnetic disk substrate 15.
In the field of jigs and tools for assembly of electronic parts, further, the electric conductivity has been required to prevent accumulation of electric charge in addition to the property of high abrasion resistance and corrosion resistance. In view thereof, the jig or tool is considered to be made of semiconductive ceramics.
However, it has been difficult to produce the above-mentioned semiconductive ceramics at low costs. For example, silicon carbide ceramics is a difficult-to-sinter material. Specifically, silicon carbide ceramics is required to be sintered at 2000.degree. C. or more in a non-oxidizing condition. In addition, the hot pressing and the hot isostatic pressing are required. As a result, the cost is high but the productivity is low.
Also, perovskite ceramics may be sintered in an oxidizing condition. However, the unit price of raw material is high. Further, perovskite ceramics generally has a bending strength less than 100 MPa, which is too low to be used as a structural material.
In production of alumina or zirconia semiconductive ceramics containing NiO, CoO or the like as a conductive provider, the hot pressing and the hot isostatic pressing are required. Further, after sintering in an oxidizing condition, another sintering is required to be conducted in a reducing condition. Such complicated production manner lowers the productivity, and makes the production control difficult. The requirement of two sinterings considerably increases the production costs.
Further, in the case of a supporting member made of the above-mentioned semiconductive ceramics, a thermal expansion difference of 2.times.10.sup.-6 /.degree. C. to 5.times.10.sup.-6 /.degree. C. occurs between the supporting member and the magnetic disk substrate 15 made of glass. Such thermal expansion difference causes a strain in the magnetic disk substrates 15, and impairs the parallelism between the magnetic disk substrates 15.
Further, as material for jigs and tools capable of removing electric charges and preventing excessive current due to electric charge, an electric conductivity of 10.sup.5 .OMEGA..multidot.cm to 10.sup.11 .OMEGA..multidot.cm in terms of volume specific resistance is required. However, alumina semiconductive ceramics added with TiO.sub.2 or TiC cannot have an electric conductivity greater than 10.sup.-2 .OMEGA..multidot.cm in terms of volume specific resistance. Accordingly, alumina semiconductive ceramics are not suitable for electric charge preventive jig or tool.